Fix error when JEMALLOC_ENABLED is not set

When loading profile.d/jemalloc we would get the below error. It boils
down to asking bash to do a string comparison when the left side wasn't
a string. The fix is to force the left side to be a string. So, if it's
unset, the comparison becomes `"" = "true"`.

```
/app/.profile.d/jemalloc.sh: line 5: [: =: unary operator expected
```

Fixes mojodna#1

bin/compile

@@ -32,7 +32,7 @@ export PATH="$PATH:/app/vendor/jemalloc/bin"
 
 # Configure LD_PRELOAD when JEMALLOC_ENABLED is set to "true". Setting this can
 # be tricky for people and it occasionally changes with new versions.
-if [ $JEMALLOC_ENABLED = 'true' ]; then
+if [ "$JEMALLOC_ENABLED" = 'true' ]; then
   export LD_PRELOAD="/app/vendor/jemalloc/lib/libjemalloc.so $LD_PRELOAD"
 fi
 EOF