Fix Concat Breaks with Double onCompleted

Fixes https://github.com/ReactiveX/RxJava/issues/1818
Merge pull request #1863 from abersnaze/backport-concat-fix

Fix Concat Breaks with Double onCompleted
1.4.+ -> 1.4.0

The + started causing issues with builds, apparently can't use "greater than".
2.+ -> 2.0.5

Need to specify exact version for build to work
1.7.+ -> 1.7.5
Update build.gradle
2.10.3
[Gradle Release Plugin] - pre tag commit:  '0.20.7'.
[Gradle Release Plugin] - new version commit:  '0.20.8-SNAPSHOT'.