TestSQLApi/TestSQLApiConn::test_database_uri_string errors if pg8000 is installed
#### Code Sample, a copy-pastable example if possible

```python
db_uri = "postgresql+pg8000://user:pass@host/dbname"
with tm.assert_raises_regex(ImportError, "pg8000"):
    sql.read_sql("select * from table", db_uri)
```
#### Problem description

The test code comments specify that `pg8000` is a "driver that will not be installed on Travis". However, when the test suite is run elsewhere, that driver may be installed.

#### Expected Output

The test should pass or skip in any environment where pandas is expected to run successfully.

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

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.13.final.0
python-bits: 64
OS: FreeBSD
OS-release: 10.3-STABLE
machine: amd64
processor: amd64
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.20.1
pytest: 3.1.0
pip: None
setuptools: 32.1.0
Cython: 0.25.2
numpy: 1.11.2
scipy: 0.19.0
xarray: None
IPython: None
sphinx: None
patsy: None
dateutil: 2.6.0
pytz: 2016.10
blosc: None
bottleneck: 1.0.0
tables: 3.4.2
numexpr: 2.6.2
feather: None
matplotlib: 1.5.3
openpyxl: 2.4.7
xlrd: 1.0.0
xlwt: 1.1.2
xlsxwriter: 0.9.6
lxml: 3.6.0
bs4: 4.5.1
html5lib: 0.9999999
sqlalchemy: 1.1.10
pymysql: 0.7.11.None
psycopg2: 2.7.1 (dt dec pq3 ext lo64)
jinja2: 2.9.5
s3fs: None
pandas_gbq: None
pandas_datareader: None
</details>
