Allow custom XGBoost versions in XGBOOST_BUILD_VERSION

Currently XGBoost.jl allow building either "master" or known stable
tags of XGBoost git repository. This change removes that restriction
and make it possible to build any tagged version, branch or git
commitsh. This is required to workaround some of XGBoost.jl package
build issues.

deps/build.jl

@@ -4,10 +4,11 @@ using BinDeps
 
 xgboost = library_dependency("xgboost", aliases = ["libxgboost.so"])
 
-if haskey(ENV, "XGBOOST_BUILD_VERSION") && ENV["XGBOOST_BUILD_VERSION"] == "master"
-    libcheckout = `git checkout master`
-    onload = "global const build_version = \"master\""
-    info("Using the latest master version of the XGBoost library")
+if haskey(ENV, "XGBOOST_BUILD_VERSION")
+    build_version = ENV["XGBOOST_BUILD_VERSION"]
+    libcheckout = `git checkout $(build_version)`
+    onload = "global const build_version = \"$(build_version)\""
+    info("Using the $(build_version) version of the XGBoost library")
 else
     libcheckout = `git checkout v0.60`
     onload = "global const build_version = \"0.60\""