Upconvert is_lax versions in conflicts during conversions

A partial solution for #55 , make sure converted meta files emit 

```json
{ 
  "conflicts": {
    "Foo" : 1
  }
}
```

And equivalent as 
```json
{ 
  "conflicts": {
    "Foo" : ">=1"
  }
}
```

To clarify the implicit semantics.

Also note: Currently ">=1" is normalised to just "1" by default with `CPAN::Meta::Requirements`

```perl
use Data::Dumper;
use CPAN::Meta::Requirements;
my $x = CPAN::Meta::Requirements->new(); 
$x->add_string_requirement( Foo => q[>=1] );
print Dumper($x->as_string_hash);
__END__
$VAR1 = {
          'Foo' => '1'
        };

```

And this issue is avoided by processing the return value from `CPAN::Meta::Requirements`.

Subsequent munging of this value outside `CPAN::Meta::Converter` will likely lose the `>=` context.

Additionally, this patch introduces a test failure I haven't understood yet:
```TAP
t/converter.t ............ 13/? 
#   Failed test 'downconversion from 2 merge test and build requirements'
#   at t/converter.t line 75.
#     Structures begin differing at:
#          $got->{Build::Requires} = Does not exist
#     $expected->{Build::Requires} = '1.1'
# Looks like you failed 1 test of 191.
```