CI: very slow tests

[jreback]

from a recent build on master, these builds are very slow. Yes we have 140k tests, but 30m is a lot of time (esp considering the azure builds are 2x faster).

needs some investigation on what is happening.

• https://travis-ci.org/github/pandas-dev/pandas/jobs/746072524
• https://travis-ci.org/github/pandas-dev/pandas/jobs/746072525

cc @pandas-dev/pandas-core

[bashtage]

Could start up upping the number of durations shown. There may be a large tranch of tests that are all not super slow but slow enough.

[ivanovmg]

pandas/tests/io/excel are pretty slow. Can gain about 5 minutes there.

[mroeschke]

Looks like we don't have the travis logs anymore. I think we can reopen a subsequent issue once the offending set of tests are found.