Start using the ci-scripts submodule

[prateek]

• Got rid of the old .ci scripts. There were 2 which were unused afaict: test.sh and build-common.sh.
• Started using the new submodule
• Some tweaks to the makefile to use the new test-cover.sh file
• Migrated env var package to travis.yml

JFYI, anyone with an existing repo will have to update their existing submodules by doing the following -

git submodule update --init --recursive 

If that doesn't work you may have to delete the folders .ci, and .git/modules/.ci manually and retry the update.

/cc @robskillington @xichen2020 @kobolog @cw9 @martin-mao @ben-lerner

[coveralls]

Coverage increased (+0.1%) to 84.781% when pulling 8ea8877 on prateek/ci-scripts-refactor into dd72a7c on master.

[xichen2020]

LGTM