Bump version number for 0.2.8 release.

gradle.properties

 GROUP=org.briarproject
 POM_ARTIFACT_ID=dont-kill-me-lib
-VERSION_NAME=0.2.7
+VERSION_NAME=0.2.8
 
 POM_NAME=Do not kill me library
 POM_DESCRIPTION=An Android library helping to keep a foreground service with wake-locks running. No other use-cases considered.