Restructure some typegen code to fix break in 1.5

[jdlangs]

As reported in #82, the code that used hardcoded accessing into Expr objects broke in julia 1.5 due to the internals of the expressions being represented differently. This gets around it by avoiding accessing Expr internals at all, preferring to splice the generated expressions where they are needed.

This also modifies travis to run on ubuntu 18.04 and ROS melodic and bumps the version to 0.8.

[coveralls]

Coverage decreased (-0.3%) to 91.628% when pulling 769b7b1 on jl1.5fix into 4df0ab0 on master.