Force RAILS_ENV to be "test" in rake tasks

After #732, if you're using the SQL schema format, `parallel:prepare`
now calls `db:purge db:structure:load` where it used to call
`db:test:prepare`. The old task forced using the test environment,
however the new tasks make no assumptions about the environment.
Therefore, when combined with a Spring-ified rake binstub, where Spring
defaults to the development environment, when `parallel:prepare` calls
`db:purge db:structure:load`, they're running in the development
environment against the development database, instead of in the test
environment against the test database.

Now the environment is forced to be "test" in all rake tasks regardless
of the RAILS_ENV that might have been set, whether by Spring or the
user. This has the side-effect of removing the ability to run the tasks
in a test-like environment that's named something other than "test".

I believe this fixes #768.

lib/parallel_tests/tasks.rb

@@ -5,7 +5,7 @@ module ParallelTests
   module Tasks
     class << self
       def rails_env
-        ENV['RAILS_ENV'] || 'test'
+        'test'
       end
 
       def rake_bin

spec/parallel_tests/tasks_spec.rb

@@ -35,13 +35,13 @@
   end
 
   describe ".rails_env" do
-    it "should be test when nothing was set" do
+    it "should be test" do
       expect(ParallelTests::Tasks.rails_env).to eq("test")
     end
 
-    it "should be whatever was set" do
+    it "should disregard whatever was set" do
       ENV["RAILS_ENV"] = "foo"
-      expect(ParallelTests::Tasks.rails_env).to eq("foo")
+      expect(ParallelTests::Tasks.rails_env).to eq("test")
     end
   end