Add a second Travis build config

@hobu mentioned in
https://github.com/PDAL/PDAL/pull/370#issuecomment-43626129 that we're a
bit lazy about our conditional file inclusion, since many PDAL developers
have relatively static Cmake configurations during development. Issues
like #370 can pop up when developers don't test out various build
configurations.

This patch adds a second Travis build configuration via the
[env](http://docs.travis-ci.com/user/build-configuration/#Set-environment-variables)
option. `.travis.yml` now sets the `PDAL_OPTIONAL_COMPONENTS`
environment variable to either "all" or "none", and this variable is
then used to control the installation of dependencies and the CMake
configuration.

I aimed to make this patch as low-impact as possible by only switching
on/off CMake variables that were either:

1. Previously controlled by `scripts/ci/script.sh` (e.g. `WITH_GDAL`)
2. Defaulted to `ON` (e.g. `WITH_ZLIB`)

All other CMake options were explicitly turned off (e.g. `WITH_SQLITE`).
These can (should?) be turned on in a later patch.