Make InstallCommandTest faster.

# Motivation
`InstallCommandTest` builds PHP multiple times and takes very long time to finish, so we need to wait for a long time to get build status on Travis CI. After applying this PR, the time of running tests on Travis CI decreases from `15 min 15 sec` to `5 min 39 sec`, for example.

# Solution
I've added `--jobs`option in `InstallCommandTest` if we can get the number of CPU processors.
would be great if we can rename this to `detectProcessorNumber()`, since this is not a pure property getter. :)