We use a separate `doc/requirements.txt` file for building the
documentation. Since on Read The Docs where we cannot install
packages depending on C libraries, we mock these packages in
`doc/conf.py` and omit them from the requirements.

Add Travis CI configuration

See merge request !26

Not sure how this came to be, but NCBI36 was incorrectly named GRCh36.
Changing this, however, breaks the sort order in assembly lists. So we
now sort on the UCSC alias (hg18).

Fixes #8

Fixes #9

Fixes #22

Fixes #30

See merge request !25