Skip fish completion tests if fish is not installed

Fixes #1654

features/support/completion.rb

@@ -107,6 +107,10 @@ setup_tmp_home = lambda { |shell|
 }
 
 $tmux = nil
+$installed_shells = Hash.new { |cache, shell|
+  `which #{shell} 2>/dev/null`
+  cache[shell] = $?.success?
+}
 
 Before('@completion') do
   unless $tmux
@@ -126,7 +130,12 @@ World Module.new {
   attr_reader :shell
 
   def set_shell(shell)
-    @shell = shell
+    if $installed_shells[shell]
+      @shell = shell
+      true
+    else
+      false
+    end
   end
 
   define_method(:tmux_pane) do
@@ -221,7 +230,7 @@ World Module.new {
 }
 
 Given(/^my shell is (\w+)$/) do |shell|
-  set_shell(shell)
+  set_shell(shell) || pending
   setup_tmp_home.call(shell)
 end