Tweak how to get BS_OS

bash/environment

@@ -54,9 +54,9 @@ _add-path /usr/local/go/bin
 _add-path ~/.bin
 _add-path $HOME/Library/Haskell/bin
 
-if [[ `uname` == 'Darwin' ]]; then
+if [[ $(uname) == 'Darwin' ]]; then
     export BS_OS='osx'
-elif [[ `uname` == 'Linux' ]]; then
+elif [[ $(uname) == 'Linux' ]]; then
     export BS_OS='linux'
 fi
 
@@ -67,4 +67,3 @@ export WORKON_HOME="$HOME/.virtualenvs"
 if [ -f /usr/local/bin/virtualenvwrapper.sh ]; then
     source /usr/local/bin/virtualenvwrapper.sh
 fi
-