Psiphon travisci build master

[jmwample]

See issue #38 and pull request #40 for most of the fixes added in this PR. These are just porting those changes into Master so that TapDance client will build.

Extra issues:

1. psiphon core library and consoleclient only build with golang 1.13+ so our build script needed to be updated to reflect that.

2. Java class issue - android is failing to build because of a class issue in the ca.psiphon.aar. See travis-ci build job #204.5 for the full error.

8: Default interface methods are only supported starting with Android N (--min-api 24): java.lang.Object ca.psiphon.PsiphonTunnel$HostService.getVpnService()

com.android.builder.dexing.DexArchiveBuilderException: com.android.builder.dexing.DexArchiveBuilderException: Failed to process /root/.gradle/caches/transforms-1/files-1.1/ca.psiphon.aar/4c609e5ebf707fc16a197763bf86e87a/jars/classes.jar

....

[CIRCULAR REFERENCE:com.android.tools.r8.ApiLevelException: Default interface methods are only supported starting with Android N (--min-api 24): java.lang.Object ca.psiphon.PsiphonTunnel$HostService.getVpnService()]

:app:transformClassesWithDexBuilderForDebug FAILED

[jmwample]

noticed that the PSIPHON_SYSTEM_VERSION was not getting set in the travis-ci build log so it was using teh out of date default I added 6 months ago. Reset default in travis-ci settings. Build succeeded once these versions were updated.