Changed String init to use Linux friendly implementation

[Drewsmits]

Couldn't start Echo server with ssl on linux due to String init method not being implemented yet. I'd get the following crash.

.build/debug/Echo serve -ssl
[".build/debug/Echo", "serve", "-ssl"]
Starting secure server
fatal error: init(contentsOf:usedEncoding:) is not yet implemented: file Foundation/NSString.swift, line 1251
Illegal instruction

Looks like the linux version of NSString.init(contentsOfFile:) hasn't been implemented yet. Looks like internally it calls NSString.init(contentsOfFile:encoding:) though, so I swapped the call.

http://stackoverflow.com/questions/41317986/string-initcontentsoffile-replacement-for-linux