[Swift4]Update templates for swift 4 (#6921)

* update versions of dependencies on swift4 and swift3

* change syntax for swift4

* run petstore script

* change enum case from UpperCamel to lowerCamel

* remove unneeded break statements

* avoid wrapping conditionals in parentheses

* avoid force casting

* run pod update on petstore/swift4/rxswift

* update project for swift 4

* run swift4-petstore-all.sh

* fix compile error

* avoid use iso8601 date strategy for encoder / decoder

* resolve file references

samples/client/petstore/swift4/objcCompatible/PetstoreClient.podspec

@@ -1,13 +0,0 @@
-Pod::Spec.new do |s|
-  s.name = 'PetstoreClient'
-  s.ios.deployment_target = '9.0'
-  s.osx.deployment_target = '10.11'
-  s.version = '0.0.1'
-  s.source = { :git => 'git@github.com:swagger-api/swagger-mustache.git', :tag => 'v1.0.0' }
-  s.authors = ''
-  s.license = 'Proprietary'
-  s.homepage = 'https://github.com/swagger-api/swagger-codegen'
-  s.summary = 'PetstoreClient'
-  s.source_files = 'PetstoreClient/Classes/**/*.swift'
-  s.dependency 'Alamofire', '~> 4.5.0'
-end