Fix broken project.clj

Slurping the project.clj file may result in a string with trailing white space (at least if I use nearly all features, that is `lein new luminus luminus-test +bootstrap +cljs +h2 +korma +site`).
This leads to a broken structure when inserting ":min-lein-version" before the 
last character. Trimming the string ensures that the last character is indeed the closing parenthesis.