Improve gamma approximation

- Use `pow()` if available (e.g. from sass-planifolia or MathSass)
- Use an approximation that is optimized to produce good contrast values

See also oddbird/accoutrement-color#6 for a
similar change.

core/bourbon/utilities/_gamma.scss

@@ -2,8 +2,7 @@
 
 /// Performs gamma correction on a single color channel.
 ///
-/// Note that Sass does not have a `pow()` function, so the calculation
-/// is approximate.
+/// Note that the calculation is approximate if a `pow()` is not available.
 ///
 /// @argument {number (0-1)} $channel
 ///
@@ -16,6 +15,10 @@
     @return $channel / 12.92;
   } @else {
     $c: ($channel + 0.055) / 1.055;
-    @return (133 * $c * $c * $c + 155 * $c * $c) / 288;
+    @if function-exists("pow") {
+      @return pow($c, 2.4);
+    } @else {
+      @return 0.56 * $c * $c * $c + 0.44 * $c * $c;
+    }
   }
 }

spec/bourbon/utilities/gamma_spec.rb

@@ -7,7 +7,7 @@
 
   context "called on a color channel" do
     it "outputs a gamma value between 0 and 1" do
-      rule = "content: 0.13185;"
+      rule = "content: 0.12168;"
 
       expect(".gamma").to have_ruleset(rule)
     end

spec/bourbon/utilities/lightness_spec.rb

@@ -23,7 +23,7 @@
 
   context "called on gray" do
     it "outputs a number between 0 and 1 to indicate lightness" do
-      rule = "content: 0.21795;"
+      rule = "content: 0.20503;"
 
       expect(".lightness-gray").to have_ruleset(rule)
     end