Make default experiment not run all try paths

It is quite possible to follow the directions in the README.md,
create a localized experiment class with a real `enabled?` method
`publish` methods, etc., and expect folks to require your library
and use it.

But, it's also possible that someone will define an experiment,
neglectd to include your local experiment override library, still
have scientist in the require path, and when they `include Scientist`
in their experiment-running class, they will get the "default"
scientist experiment.

The default experiment runs every `try` branch.

In production environments pushing a new experiment and having it
run the `try` branch unconditionally is less than ideal.

This flips the default `enabled?` to 'false`.  At least folks will
dig a bit to realize they could include their local library by
default and dodge this problem.

Author: rick

lib/scientist/default.rb

@@ -10,9 +10,9 @@ def initialize(name)
     @name = name
   end
 
-  # Run everything every time.
+  # Don't run experiments.
   def enabled?
-    true
+    false
   end
 
   # Don't publish anything.

test/scientist/default_test.rb

@@ -3,8 +3,8 @@
     @ex = Scientist::Default.new "default"
   end
 
-  it "is always enabled" do
-    assert @ex.enabled?
+  it "is always disabled" do
+    refute @ex.enabled?
   end
 
   it "noops publish" do

test/scientist/experiment_test.rb

@@ -123,6 +123,11 @@ def publish(result)
   it "re-raises exceptions raised during publish by default" do
     ex = Scientist::Experiment.new("hello")
     assert_kind_of Scientist::Default, ex
+
+    def ex.enabled?
+      true
+    end
+
     def ex.publish(result)
       raise "boomtown"
     end