BUG: IntervalIndex.get_loc fails when there is only one entry
#### Code Sample, a copy-pastable example if possible

```python
>>> import pandas as pd
>>> pd.IntervalIndex.from_tuples([(1,100)]).get_loc(50)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/imre/code/pandas/pandas/core/indexes/interval.py", line 1049, in get_loc
    raise KeyError(original_key)
KeyError: 50
```
#### Problem description
50 is contained in the interval (1, 100), so this should not raise `KeyError`.

#### Expected Output
0

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit: c4da79b5b322c73d8e61d1cb98ac4ab1e2438b40
python: 3.6.3.final.0
python-bits: 64
OS: Linux
OS-release: 4.13.0-39-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.23.0.dev0+824.gc4da79b5b
pytest: None
pip: 10.0.1
setuptools: 39.1.0
Cython: 0.28.2
numpy: 1.14.3
scipy: None
pyarrow: None
xarray: None
IPython: None
sphinx: None
patsy: None
dateutil: 2.7.2
pytz: 2018.4
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
