Add pager detection by using `which` command

Fix issue #743 (pager gives warnings `Missing filename ("less --help"
for help)`)

Signed-off-by: Kyrylo Silin <kyrylosilin@gmail.com>

lib/pry/pager.rb

@@ -51,7 +51,9 @@ class Pry::Pager
     end
 
     def self.available?
-      `#{default_pager}`rescue nil
+      pager_executable = default_pager.split(' ').first
+      `which #{ pager_executable }`
+    rescue
     end
 
     def initialize(*)