patch up zshrc to work on other computers

dotfiles/zsh/exports.zsh

@@ -29,5 +29,10 @@ export HISTCONTROL=ignoredups
 # If go is installed, set the $GOPATH to the projects directory created above.
 if which go > /dev/null; then export GOPATH=$HOME/.go; fi
 	
-export PATH="$PATH:$HOME/.rvm/bin"
-. "$HOME/.cargo/env"
+if [ -d "$HOME/.rvm" ]; then
+	export PATH="$PATH:$HOME/.rvm/bin"
+fi
+
+if [ -f "$HOME/.cargo/env" ]; then
+	. "$HOME/.cargo/env"
+fi

dotfiles/zsh/zshrc

@@ -59,11 +59,11 @@ if [[ ! -d $HOME/.zsh ]]; then
   popd
 fi
 
+autoload -U is-at-least
+
 # Install zsh-syntax-highlighting if it doesn't exist
-if [[ ! -d $HOME/.zsh/zsh-syntax-highlighting ]]; then 
-  if ! is-at-least 4.3.11; then
-    echo "ZSH syntax highlighting requires ZSH >= 4.3.11 (current: $ZSH_VERSION)."
-  else
+if is-at-least 4.3.11; then
+	if [[ ! -d $HOME/.zsh/zsh-syntax-highlighting ]]; then 
 	    echo "Installing zsh syntax highlighting"
 	    pushd
 	    
@@ -72,7 +72,8 @@ if [[ ! -d $HOME/.zsh/zsh-syntax-highlighting ]]; then
 
 	    popd
   	fi
-
+else
+	echo "ZSH syntax highlighting requires ZSH >= 4.3.11 (current: $ZSH_VERSION)."
 fi 
 
 zstyle ':completion:*:*:git:*' script $HOME/.zsh/git-completion.bash