Fixed File::IsOpen() returning false for APK files on Android. Closes #179.
Fix AutoExposure's log luminance calculation, add LumRange parameter and remove Sensitivity parameter.
For Travis CI - Install rake into global gem set.

It was not neccessary before but somehow all the Travis CI build environment for C++ language today do not have rake pre-installed anymore.
Sync at 2014-02-08 17:14:13 +0800.