Fix issues with fields that are xs:simpleType.

There were two issues.

1) PHP doc for enum properties was wrong.

@property DTS\eBaySDK\Trading\Types\EnumStringType(string) $enumString

This is now corrected.

@property DTS\eBaySDK\Trading\Enums\EnumStringType(string) $enumString

2) Some eBay types are derived from xs:simpleType. XSLT was incorrectly
creating these as enums. These fields don't need to be a PHP class as
they can be represented with simple PHP types.
Merge branch 'simple-types' into develop
Ensure composer updates when running in travis.
Merge branch 'travis' into develop