Travis build fails for android api level < 19

[Timothée Jeannin]

See https://travis-ci.org/TimotheeJeannin/ProviGen/builds/18229844

[INFO] ProviGen - parent ................................. SUCCESS [0.891s]
[INFO] ProviGen .......................................... SUCCESS [3.395s]
[INFO] ProviGen - Sample ................................. FAILURE [0.208s]
[INFO] ProviGen - tests .................................. SKIPPED
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 7.657s
[INFO] Finished at: Tue Feb 04 21:50:51 UTC 2014
[INFO] Final Memory: 19M/186M
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal com.jayway.maven.plugins.android.generation2:android-maven-plugin:3.8.1:generate-sources (default-generate-sources) on project ProviGen-sample: Execution default-generate-sources of goal com.jayway.maven.plugins.android.generation2:android-maven-plugin:3.8.1:generate-sources failed: Invalid SDK: Platform/API level 19 not available. This command should give you all you need:
[ERROR] /home/travis/build/TimotheeJeannin/ProviGen/android-sdk-linux/tools/android update sdk --no-ui --obsolete --force