Decrease minimum SDK version

[sampalmer]

The library's minimum SDK version is 15. This causes an error when the library is included in an app with a lower minimum version. The library code relies on the support library, so I wouldn't expect it to actually need an API level this high.

Would you consider lowering it to 14 or lower?

[dmitvitalii]

@sampalmer you can use this fork https://github.com/yotadevices/RtlViewPager with min API 13 for now, 'till (and if) it will be fixed and updated for current diego-gomez-olvera/RtlViewPager.