Fix errors and upgrade to ubuntu:18.04

[maresb]

I was getting the following error when building lemur-web:

Collecting black==19.3b0 (from lemur[tests])
  ERROR: Could not find a version that satisfies the requirement black==19.3b0 (from lemur[tests]) (from versions: none)
ERROR: No matching distribution found for black==19.3b0 (from lemur[tests])
make: *** [develop] Error 1
Makefile:7: recipe for target 'develop' failed
ERROR: Service 'lemur-web' failed to build: The command '/bin/sh -c git config --global url."https://".insteadOf git:// &&  cd /usr/local/src &&  git clone https://github.com/netflix/lemur.git &&  cd lemur &&  git checkout ${LEMUR_VERSION} &&  pip install --upgrade pip virtualenv &&  export PATH=/usr/local/src/lemur/venv/bin:${PATH} &&  virtualenv -p python3 venv &&  . venv/bin/activate &&  make ${LEMUR_TARGET}' returned a non-zero code: 2

This seemed to be related to the inclusion of the libssl-dev package in the apt command. Then there was an issue with ubuntu:16.04 packaging python3.5, which doesn't work since lemur uses f-strings. Thus I updated Ubuntu to 18.04. Finally, there was a problem involving postgres permissions to create the pg_trgm extension, preventing the user table from being created. Thus I made the postgres user a superuser.

Those were all of the changes I made, and now it works for me.

[maresb]

I suspect that this resolves Issue #37.

[drewby08]

Looks good! I whipped up a 16.04 fix before I saw you managed to fix it using 18.04: https://github.com/Netflix/lemur-docker/pull/40

[kevgliss]

Thanks for the PR!