Fix podspec source specification (#26)

Fixes #25. Ruby only interpolates values in double-quoted strings

Author: rolodato

nodejs-mobile-react-native.podspec

@@ -7,7 +7,7 @@ Pod::Spec.new do |s|
   s.version      = package['version']
   s.summary      = package['description']
   s.license      = package['license']
-  s.source       = { :git => 'https://github.com/nodejs-mobile/nodejs-mobile-react-native.git', :tag => 'v#{s.version}' }
+  s.source       = { :git => 'https://github.com/nodejs-mobile/nodejs-mobile-react-native.git', :tag => "v#{s.version}" }
   s.authors      = package['author']
   s.homepage     = package['homepage']
   s.platform     = :ios, '9.0'
@@ -21,4 +21,4 @@ Pod::Spec.new do |s|
   s.ios.vendored_frameworks = 'ios/NodeMobile.framework'
   s.static_framework = true
   s.dependency 'React-Core'
-end
+end