Quantile result errors.
#### Code Sample, a copy-pastable example if possible

```python
# Your code here
>>> loc1series4 = pd.Series([6,8,10,12,14,9,11,7,13,11])
```
#### Problem description
>>> loc1series4.quantile(0.25)
8.25
>>> loc1series4.quantile(0.75)
11.75
Wrong result.
[this should explain **why** the current behaviour is a problem and why the expected output is a better solution.]

#### Expected Output
>>> loc1series4.quantile(0.25)
7.75
>>> loc1series4.quantile(0.75)
12.25
#### Output of ``pd.show_versions()``

<details>
# Paste the output here pd.show_versions() here
>>> pd.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 3.5.2.final.0
python-bits: 64
OS: Darwin
OS-release: 16.3.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_GB.UTF-8
LOCALE: en_GB.UTF-8

pandas: 0.19.1
nose: None
pip: 9.0.1
setuptools: 27.2.0
Cython: None
numpy: 1.11.2
scipy: None
statsmodels: None
xarray: None
IPython: None
sphinx: None
patsy: None
dateutil: 2.6.0
pytz: 2016.10
blosc: None
bottleneck: None
tables: None
numexpr: None
matplotlib: 1.5.3
openpyxl: None
xlrd: 1.0.0
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
boto: None
pandas_datareader: None
</details>