update import paths in sub-READMEs

closes #113

ast/README.markdown

@@ -1,6 +1,6 @@
 # ast
 --
-    import "github.com/robertkrimen/otto/ast"
+    import "github.com/dop251/goja/ast"
 
 Package ast declares types representing a JavaScript AST.
 

file/README.markdown

@@ -1,6 +1,6 @@
 # file
 --
-    import "github.com/robertkrimen/otto/file"
+    import "github.com/dop251/goja/file"
 
 Package file encapsulates the file abstractions used by the ast & parser.
 

token/README.markdown

@@ -1,6 +1,6 @@
 # token
 --
-    import "github.com/robertkrimen/otto/token"
+    import "github.com/dop251/goja/token"
 
 Package token defines constants representing the lexical tokens of JavaScript
 (ECMA5).