httpie: the build input 'requests014' doesn't exist anymore, it's now called 'requests'
httpie: update to version 0.4.1 to fix the build
Switch stdenv to GCC 4.8.0.
gmp-4.3.2.nix: disable test suite to fix the build

The test t-lucnum_ui fails (on Linux/x86_64) when built with GCC 4.8.
Newer versions of GMP don't have that issue anymore.