travis: Don't build python bindings in travis

Due to rust-lang/rust#55380, travis cannot
compile pyportus. These failures mask other real errors. Therefore,
temporarily disable travis builds of the python bindings until either:
1. we can specify rust nightly version in travis
2. ICE goes away.

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")