Add a travis script

Add a build script and ask Travis to run it. It downloads the tip of
libgit2's dev branch and tests against that.

.travis.yml

@@ -0,0 +1,12 @@
+language: go
+
+go:
+  - 1.0
+  - 1.1
+  - tip
+
+env:
+  - PKG_CONFIG_PATH=libgit2/install/lib/pkgconfig LD_LIBRARY_PATH=libgit2/install/lib
+
+install:
+  - script/build-libgit2.sh

script/build-libgit2.sh

@@ -0,0 +1,17 @@
+#!/bin/sh
+
+set -ex
+
+git clone --depth 1 --single-branch git://github.com/libgit2/libgit2 libgit2
+
+cd libgit2
+cmake -DTHREADSAFE=ON \
+      -DBUILD_CLAR=OFF \
+      -DCMAKE_INSTALL_PREFIX=$PWD/install \
+      .
+
+make install
+
+# Let the Go build system know where to find libgit2
+export LD_LIBRARY_PATH="$TMPDIR/libgit2/install/lib"
+export PKG_CONFIG_PATH="$TMPDIR/libgit2/install/lib/pkgconfig"