title: R reticulate unable to find installed python library
body: <p>I'm venturing into using Reticulate in R and having trouble installing a package, specifically <code>psycopg2</code> but I've also tried installing <code>twisted</code> with the same result. </p>
  
  <p>after I load <code>reticulate</code> in R I double check to make sure my package is installed:</p>
  
  <pre><code>&gt; conda_install(envname = "r-reticulate", packages="psycopg2")
  Solving environment: ...working... done
  
  # All requested packages already installed.
  </code></pre>
  
  <p>Looks good. So I set my condaenv to <code>r-reticulate</code> just to be double sure. The docs say I should not have to do this, but it should not hurt:</p>
  
  <pre><code>&gt; use_condaenv( "r-reticulate")
  </code></pre>
  
  <p>Then I try to import <code>psycopg2</code>:</p>
  
  <pre><code>&gt; psycopg2 &lt;- import('psycopg2')
  Error in py_module_import(module, convert = convert) : 
    ImportError: No module named psycopg2
  </code></pre>
  
  <p>If I install <code>twisted</code> the same way, I get the same error:</p>
  
  <pre><code>&gt; twisted &lt;- import('twisted')
  Error in py_module_import(module, convert = convert) : 
    ImportError: No module named twisted
  </code></pre>
  
  <p>So, what am I doing wrong here?</p>
  
  <p>Below is the output of <code>conda_list</code> and <code>py_config</code>... I was expecting <code>py_config</code> to show me the path to the python exe in my <code>r-reticulate</code> environment. However I don't have anything to compare this to, so my expectations may have been wrong. </p>
  
  <pre><code>&gt; conda_list()
            name                                                                              python
  1     Anaconda                     C:\\Users\\jal\\AppData\\Local\\Continuum\\Anaconda\\python.exe
  2 r-reticulate C:\\Users\\jal\\AppData\\Local\\Continuum\\Anaconda\\envs\\r-reticulate\\python.exe
  
  &gt; py_config()
  python:         C:\Users\jal\AppData\Local\CONTIN~1\Anaconda\python.exe
  libpython:      C:/Users/jal/AppData/Local/CONTIN~1/Anaconda/python27.dll
  pythonhome:     C:\Users\jal\AppData\Local\CONTIN~1\Anaconda
  version:        2.7.15 |Anaconda custom (64-bit)| (default, May  1 2018, 18:37:09) [MSC v.1500 64 bit (AMD64)]
  Architecture:   64bit
  numpy:          C:\Users\jal\AppData\Local\CONTIN~1\Anaconda\lib\site-packages\numpy
  numpy_version:  1.14.3
  pandas:         C:\Users\jal\AppData\Local\CONTIN~1\Anaconda\lib\site-packages\pandas
  
  python versions found: 
   C:\Users\jal\AppData\Local\CONTIN~1\Anaconda\python.exe
   C:\Users\jal\AppData\Local\CONTIN~1\ANACON~1\python.exe
  </code></pre>