BUG: fix metacharacter replacement bug in DataFrame.replace()
closes #6777

merge in green
 on
 blah yahoo junk is failing again
 @jreback any idea why this doesn't say green even tho https://travis-ci.org/cpcloud/pandas/builds/22297556 is all green?
 it turns green after the build on pydata/pandas runs (and not cpcloud/pandas)

essentially it's run twice (I think this is a Travis thing)

just wait a bit
 ah ok thanks ... green now bombs away
