tab: clear Formula.installed cache when created.

This was causing the flaky tests that #1508 started skipping.

This is the second time that `Formula.installed`'s cache has bitten
me with intermittent test failures, and I'd like it to be the last,
so I've made it so the cache is cleared automatically when a tab is
created. This _should_ mean that the cache is cleared any time it
needs to be, with the exception of when a Keg is created artificially
with no tab. I don't think there's anything I can do to automatically
handle that use-case, though.

Library/Homebrew/tab.rb

@@ -317,6 +317,10 @@ class Tab < OpenStruct
   end
 
   def write
+    # If this is a new installation, the cache of installed formulae
+    # will no longer be valid.
+    Formula.clear_cache unless tabfile.exist?
+
     CACHE[tabfile] = self
     tabfile.atomic_write(to_json)
   end

Library/Homebrew/test/keg_test.rb

@@ -327,7 +327,6 @@ class InstalledDependantsTests < LinkTests
     f = stub_formula_name(name)
     keg = super
     Tab.create(f, DevelopmentTools.default_compiler, :libcxx).write
-    Formula.clear_cache
     keg
   end