fix(installation): Provide a backup git reference when tag can't be curl

closes rust-lang#423

If the parsed JSON data curled during a bash installation is not valid, use the repository's tag files
as a backup. If those files don't exist somehow, then checkout the master branch and install it.

install.sh

@@ -102,12 +102,30 @@ Path=${1:-rustlings/}
 echo "Cloning Rustlings at $Path..."
 git clone -q https://github.com/rust-lang/rustlings $Path
 
+cd $Path
+
 Version=$(curl -s https://api.github.com/repos/rust-lang/rustlings/releases/latest | ${PY} -c "import json,sys;obj=json.load(sys.stdin);print(obj['tag_name']);")
 CargoBin="${CARGO_HOME:-$HOME/.cargo}/bin"
 
+if [[ -z ${Version} ]]
+then
+    echo "The latest tag version could not be fetched remotely."
+    echo "Using the local git repository..."
+    Version=$(ls -tr .git/refs/tags/ | tail -1)
+    if [[ -z ${Version}  ]]
+    then
+        echo "No valid tag version found"
+        echo "Rustlings will be installed using the master branch"
+        Version="master"
+    else
+        Version="tags/${Version}"
+    fi
+else
+    Version="tags/${Version}"
+fi
+
 echo "Checking out version $Version..."
-cd $Path
-git checkout -q tags/$Version
+git checkout -q ${Version}
 
 echo "Installing the 'rustlings' executable..."
 cargo install --force --path .