Use a tighter grep to avoid dnvm.sh conflicts.

Fixes #838.

install.sh

@@ -240,7 +240,7 @@ nvm_do_install() {
     printf "$SOURCE_STR"
     echo
   else
-    if ! grep -qc 'nvm.sh' "$NVM_PROFILE"; then
+    if ! command grep -qc '/nvm.sh' "$NVM_PROFILE"; then
       echo "=> Appending source string to $NVM_PROFILE"
       printf "$SOURCE_STR\n" >> "$NVM_PROFILE"
     else