fixed build.gradle for the environment w/o vars 'sonatype.username' and 'sonatype.password'

Problem: when compiling in the environment where variables 'sonatype.username' and 'sonatype.password' are not defined, gradle returns an error: 

FAILURE: Build failed with an exception.
* Where:
Build file '/home/ahi/Projects/gradle-retrolambda/build.gradle' line: 52
* What went wrong:
A problem occurred evaluating root project 'gradle-retrolambda'.
> Could not find property 'sonatype.username' on root project 'gradle-retrolambda'.

This fix protects against this error, effectively allowing to compile on "vanilla" machines.