travis: Ensure cargo links libcurl statically
travis: Ensure cargo links libcurl statically

We don't want a dynamic dependency in the library that we ship, so link it
statically by configuring curl-sys's build script to not pick up the system
version via pkg-config.

graydon/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,aidancully/rust,aidancully/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,graydon/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,graydon/rust,aidancully/rust