Moves to 2018.08 references to 2018.07

Since 2018.07 is not really going to happen https://twitter.com/zoffix/status/1023294048268967937

Author: JJ

doc/Language/terms.pod6

@@ -238,7 +238,7 @@ by default.
         constant %bar = %(:42foo, :100bar);
     =end code
 
-I<NOTE: if you're using the Rakudo compiler, you need version 2018.07
+I<NOTE: if you're using the Rakudo compiler, you need version 2018.08
 or newer for type constraints and auto-coercion on constants to be available.
 Auto-coercion on %-sigilled constants requires 6.d language, preview version
 of which can be enabled with the C«use v6.d.PREVIEW» pragma>

doc/Type/Attribute.pod6

@@ -62,7 +62,8 @@ error.
     CATCH{ default { say .^name, ': ', .Str } }
     # OUTPUT: «X::Attribute::Required: The attribute '$!a' is required, but you did not provide a value for it.␤»
 
-B<Available as of 6.d language version> (early implementation exists in Rakudo compiler 2018.07+):
+B<Available as of 6.d language version> (early implementation exists in Rakudo
+compiler 2018.08+):
 
 You can specify a reason why the attribute is required:
 

doc/Type/IO.pod6

@@ -256,7 +256,7 @@ this is equivalent to printing C<$msg> to
 L<STDOUT|https://en.wikipedia.org/wiki/Standard_streams#Standard_output_.28stdout.29>, reading a line from
 L<STDIN|https://en.wikipedia.org/wiki/Standard_streams#Standard_input_.28stdin.29>,
 removing the trailing new line, and returning the resultant string. As of
-Rakudo 2018.07, C<prompt> will create L<allomorphs|/language/numerics#Allomorphs>
+Rakudo 2018.08, C<prompt> will create L<allomorphs|/language/numerics#Allomorphs>
 for numeric values, equivalent to calling C<val prompt>.
 
 =for code

doc/Type/IO/Handle.pod6

@@ -911,7 +911,7 @@ L<TTY|https://en.wikipedia.org/wiki/Terminal_emulator>, C<False> otherwise.
 =head1 Creating Custom Handles
 
 As of 6.d language (early implementation available in Rakudo compiler
-version 2018.07), a few helper methods are available to simplify creation of
+version 2018.08), a few helper methods are available to simplify creation of
 custom C<IO::Handle> objects. In your subclass you simply need to implement
 those methods to affect all of the related features. If your handle wants
 to work with textual read/write methods and doesn't use the standard C<.open>