travis: Don't build python bindings in travis

Due to rust-lang/rust#55380, travis cannot
compile pyportus. These failures mask other real errors.

Could not pin travis builds to rust nightly-2018-10-23 until ICE is resolved (or
pyo3 0.5.0 is released, which will resolve this).

Disable travis builds of python bindings temporarily.

Makefile

@@ -1,6 +1,6 @@
 all: build test-portus test-ipc libccp-integration lint
 
-travis: build test-portus libccp-integration bindings
+travis: build test-portus libccp-integration
 
 OS := $(shell uname)
 CLIPPY := $(shell rustup component list --toolchain nightly | grep "clippy" | grep -c "installed")