TST: Do not skip MySQL tests on Travis.
A first step for #4163

The build [failed](https://travis-ci.org/danielballan/pandas/jobs/8892378) because of something in `pandas.io.tests.test_data.TestYahoo`. Should I just try pushing again? Previous test on the [exact same code passed](https://travis-ci.org/danielballan/pandas/builds/8891299).

The error is small, and moreover it's unrelated to my changes.

```
AssertionError: 
Arrays are not almost equal to 5 decimals expected 904.91000 but got 904.90000
```
 rebase off of master and try again, i just merged a commit to fix that
 Full speed ahead.
 [2.7 build](https://travis-ci.org/danielballan/pandas/jobs/8895481) sulking a little:

```
/home/travis/virtualenv/python2.7_with_system_site_packages/local/lib/python2.7/site-packages/pandas-0.12.0rc1-py2.7-linux-x86_64.egg/pandas/io/tests/test_sql.py:391: Warning: Unknown table 'test_table'
  cur.execute(drop_sql)
/home/travis/virtualenv/python2.7_with_system_site_packages/local/lib/python2.7/site-packages/pandas-0.12.0rc1-py2.7-linux-x86_64.egg/pandas/io/tests/test_sql.py:408: Warning: Unknown table 'test_table2'
  cur.execute(drop_sql)
./home/travis/virtualenv/python2.7_with_system_site_packages/local/lib/python2.7/site-packages/pandas-0.12.0rc1-py2.7-linux-x86_64.egg/pandas/io/tests/test_sql.py:300: Warning: Unknown table 'test'
  cur.execute(drop_sql)
```
 Is this running all the tests now? Merge?
 Merge.
 does this also correct skip when mysql not installed?
 Yes.
