travis: keep installing our custom libcurl build

The block in the script installs the packages if we're _not_ on Precise. This
was dropped in c17c3f8a ("travis: drop support for Ubuntu Precise") in error.

script/install-deps-linux.sh

@@ -2,4 +2,11 @@
 
 set -x
 
+if [ -z "$PRECISE" ]; then
+    echo "deb http://libgit2deps.edwardthomson.com trusty libgit2deps" | sudo tee -a /etc/apt/sources.list
+    sudo apt-key adv --keyserver pgp.mit.edu --recv 99131CD5
+    sudo apt-get update -qq
+    sudo apt-get install -y curl libcurl3 libcurl3-gnutls libcurl4-gnutls-dev
+fi
+
 sudo apt-get install -y cmake libssh2-1-dev openssh-client openssh-server valgrind