FIX/ENH: attempt soft conversion of object series before raising a TypeError when plotting
DOC: add release notes/whatsnew
CLN: move PerformanceWarning to io.common and clean up
ENH/CLN: use @jtratner's decorator to check for raised warnings
Merge pull request #3912 from cpcloud/try-convert-when-plotting

FIX/ENH: attempt soft conversion of object series before raising a TypeError when plotting
TST: Fix error in assert_produces_warning.

When I copied this over and sent to @cpcloud,
I thought the indentation was wrong and changed it.
Turned out I was wrong -- sorry about that!

Now all the doctests pass...
Merge pull request #3927 from jtratner/fix-indentation-error-in-assert-produces-warning

TST: Fix error in assert_produces_warning.
CLN: remove unused travis network build cache, simplify ci/
DOC: fix to_json docstring nesting issue
Merge pull request #3930 from cpcloud/json-docstring-fix

DOC: fix to_json docstring nesting issue
Merge pull request #3929 from y-p/PR_PTF_no_more

PTF no more