fromString(): version is used outside of block scope

[mscudlik]

In method fromString() the version variable is used in line 147 to construct a new object,
but the variable is declared with a block scope inside an if statement (line 123).

This is an issue when using packageurl-js from typescript:
https://www.typescriptlang.org/docs/handbook/variable-declarations.html#block-scoping

As a consequence, fromString() always throws an error using typescript:

message:"version is not defined"
stack:"ReferenceError: version is not defined

[steven-esser]

@mscudlik Would #8 fix this?