Fix Travis Builds.

[aadrian]

Fix Travis Builds by:

• using a Travis build matrix
• updating to the latest JDKs for each version
• using more memory for Java 8 (since the tests seem to give OutOfMemory with default settings on Travis) - Java 7 has no problem whatsoever.

[grandinj]

the VM -Xmx setting is actually set inside the org.h2.build.Build class in the test(boolean) method

Note that if you bump it up too much, some tests will take longer because they deliberately trigger a out of memory condition to test the failure paths for that case

[aadrian]

@grandinj If I'm not setting it for JDK 8 at all (like I'm not doing it for Java
7), it won't pass the tests at all, since it always gives out of memory with the default size Travis-CI is setting:

• Java 8 https://travis-ci.org/aadrian/h2database/jobs/239279995
• Java 7 https://travis-ci.org/aadrian/h2database/jobs/239279996 - always without problems.

After increasing only for the Java 8 the memory, it's better:

• https://travis-ci.org/aadrian/h2database/builds/239285228

but still not perfect, since for Java 8 there seems to have random errors when running them at different times:
https://travis-ci.org/aadrian/h2database/jobs/239377022

I tried to run this test battery (what TravisCI is running) on different machines with Java 8, and in 60% of the cases one of the tests if failing. Never with Java 7.