travis: only install custom libcurl on trusty

Author: Edward Thomson

script/install-deps-linux.sh

@@ -2,7 +2,11 @@
 
 set -x
 
-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 cmake curl libcurl3 libcurl3-gnutls libcurl4-gnutls-dev libssh2-1-dev openssh-client openssh-server valgrind
+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