chore: simplify .travis.yml dependencies installation across OSes (#938)
chore: simplify .travis.yml dependencies installation across OSes (#938)

After our recent changes, the command to install dependencies for both
OS X and GNU/Linux has been unified as `make electron-develop`, so there
is no longer a need to have multiple conditionals for each OS in
`.travis.yml`.

Signed-off-by: Juan Cruz Viotti <4148f5869aa1bcd7efd85b513a5874967e585032@openmailbox.org>
resin-io/resin-etcher,resin-io/resin-etcher,resin-io/etcher,resin-io/etcher,resin-io/herostratus,resin-io/etcher,resin-io/herostratus,resin-io/etcher,resin-io/etcher