Fix Clang warning about unhandled enum values.

Clang had been generating the following warning:

```
ParsedExpression.cpp:123:13:
warning:
      30 enumeration values not handled in switch: 'CONSTANT', 'VARIABLE',
      'CUSTOM'...
            [-Wswitch]
                switch (node.getOperation().getId()) {
                                ^
                                1 warning generated.
```

To get rid of this warning, I added a default case, which does nothing.
In travis, use multiple cores.

Travis VM's have up through 2 cores. Passing a -j2 flag should make/ctest the builds/tests faster.
Merge pull request #486 from chrisdembia/travis-parallel-build

In travis, use multiple cores.
Merge pull request #485 from chrisdembia/fix-parsed-expression-warning

Fix Clang warning about unhandled enum values.