linux-x64: use a newer cmake

libgit2 now requires a cmake from within the most recent half decade.
Use a newer version during the build.

Author: ethomson

Dockerfile.linux-x64

@@ -2,6 +2,8 @@ FROM ubuntu:14.04
 WORKDIR /nativebinaries
 COPY . /nativebinaries/
 
+RUN apt update && apt -y install software-properties-common
+RUN add-apt-repository ppa:george-edison55/cmake-3.x
 RUN apt update && apt -y install cmake libssl-dev pkg-config
 
 CMD ["/bin/bash", "-c", "./build.libgit2.sh"]