Look for .api file in a case-insensitive manner

[qwwdfsad]

Fixes #76

[ilya-g]

Maybe it's worth to log a warning when the case of an api file name doesn't match the expected case?
Could apiDump task be changed to normalize the case of the file name?

[qwwdfsad]

Sorry, thought we've discussed it on meeting, should've written the rationale anyway.

The problem with mixed case sensitive/case-insensitive is that there is no single source of truth -- if the project is being developed exclusively on OS X, than CamelProjectCase can be preferable over normalized file name. Normalizing it will also trigger disruptive change for everyone updated, which isn't a good thing.

Also, the original issue (slackhq/EitherNet#52) is really weird and it's hard to tell whether a single change in minor Gradle version leading to changing the case of project name in the Gradle model a desired behaviour or a bug and triggering a warning over such thing doesn't seem reasonable to me.

[ZacSweers]

unfortunately it looks like it still fails in 0.9.0 :/. Same PR

[ZacSweers]

part of the problem is that it uses the project name (which has capital letters). If I regenerate it with all lowercase, then it works. It does seem to expect it to be all lowercase though when checking