Fix bootsnap setup in environments without bundler

Fix: #488

`ExplicitRequire.with_gems` assumed the provided gems were activated
which is only true if `bundler/setup` was required.

If bootsnap is used without bundler, then we need to explictly
activate the gem before mutating the `$LOAD_PATH`, otherwise
the paths appended during gems activation will be lost once we
exit `with_gems`.

lib/bootsnap/explicit_require.rb

@@ -25,6 +25,11 @@ def self.from_archdir(feature)
     # This is useful before bootsnap is fully-initialized to load gems that it
     # depends on, without forcing full LOAD_PATH traversals.
     def self.with_gems(*gems)
+      # Ensure the gems are activated (their paths are in $LOAD_PATH)
+      gems.each do |gem_name|
+        gem gem_name
+      end
+
       orig = $LOAD_PATH.dup
       $LOAD_PATH.clear
       gems.each do |gem|