Performance: Schema cache speedup.

[jensens]

1. in schema cache, By first checking for string, the providedBy can be avoided. Since this function is called really often it makes a difference. According to my profiling its speeds up plone/dexterity/schema.py function decorator by factor ~1.54x.
2. in schema cache, the FTI itself can be cached on the request. Using the cache is twice as fast as the lookup itself.
3. fix travis setup to test 5.0 to 5.2 and py2/py3
4. remove 2 deprecation warnings from test

Tthis is a part of #113, split to not to do too much at once.

[jensens]

@jenkins-plone-org please run jobs

[jensens]

@jenkins-plone-org please run jobs

[jensens]

@jenkins-plone-org please run jobs