title: ImportError: No module named json when using Kalibr
body: <p>I am trying Kalibr to calibrate my camera.</p>
  <p>When I run this command: ./kalibr_calibrate_cameras --target dynamic/april_6x6.yaml --bag dynamic/dynamic.bag --modules pinhole-equi --topics /cam0/image_raw</p>
  <p>I always get ImportError: No module named json. I am not very familiar with python. I am not sure where is wrong.</p>
  <p>When I try</p>
  <pre><code>$ python
  import json
  </code></pre>
  <p>It works well.
  I also try to run the file that needs json here. Still, no issues found.</p>
  <pre><code>importing libraries
  Traceback (most recent call last):
    File &quot;/var/kalibr-build/devel/bin/kalibr_calibrate_imu_camera&quot;, line 5, in &lt;module&gt;
      exec(fh.read())
    File &quot;&lt;string&gt;&quot;, line 3, in &lt;module&gt;
    File &quot;/var/kalibr-build/devel/lib/python2.7/dist-packages/sm/__init__.py&quot;, line 34, in &lt;module&gt;
      exec(__fh.read())
    File &quot;&lt;string&gt;&quot;, line 5, in &lt;module&gt;
    File &quot;/var/kalibr-build/src/kalibr/Schweizer-Messer/sm_python/python/sm/plotCoordinateFrame.py&quot;, line 2, in &lt;module&gt;
      import pylab as p
    File &quot;/home/ethan/.local/lib/python2.7/site-packages/pylab.py&quot;, line 1, in &lt;module&gt;
      from matplotlib.pylab import *
    File &quot;/home/ethan/.local/lib/python2.7/site-packages/matplotlib/__init__.py&quot;, line 141, in &lt;module&gt;
      from ._version import get_versions
    File &quot;/home/ethan/.local/lib/python2.7/site-packages/matplotlib/_version.py&quot;, line 7, in &lt;module&gt;
      import json
  ImportError: No module named json
  Error in sys.excepthook:
  Traceback (most recent call last):
    File &quot;/usr/lib/python2.7/dist-packages/apport_python_hook.py&quot;, line 64, in apport_excepthook
      from apport.fileutils import likely_packaged, get_recent_crashes
  ImportError: No module named apport.fileutils
  
  Original exception was:
  Traceback (most recent call last):
    File &quot;/var/kalibr-build/devel/bin/kalibr_calibrate_imu_camera&quot;, line 5, in &lt;module&gt;
      exec(fh.read())
    File &quot;&lt;string&gt;&quot;, line 3, in &lt;module&gt;
    File &quot;/var/kalibr-build/devel/lib/python2.7/dist-packages/sm/__init__.py&quot;, line 34, in &lt;module&gt;
      exec(__fh.read())
    File &quot;&lt;string&gt;&quot;, line 5, in &lt;module&gt;
    File &quot;/var/kalibr-build/src/kalibr/Schweizer-Messer/sm_python/python/sm/plotCoordinateFrame.py&quot;, line 2, in &lt;module&gt;
      import pylab as p
    File &quot;/home/ethan/.local/lib/python2.7/site-packages/pylab.py&quot;, line 1, in &lt;module&gt;
      from matplotlib.pylab import *
    File &quot;/home/ethan/.local/lib/python2.7/site-packages/matplotlib/__init__.py&quot;, line 141, in &lt;module&gt;
      from ._version import get_versions
    File &quot;/home/ethan/.local/lib/python2.7/site-packages/matplotlib/_version.py&quot;, line 7, in &lt;module&gt;
      import json
  ImportError: No module named json
  </code></pre>