Fix gatsby-remark-prismjs highlight bug

[vincentbel]

Resolves #1917.

I use gatsby-dev-cli to test it in the demo repo, and it works now.

At first I forgot to clear the cache, and it drove me crazy. 😹

[vincentbel]

I'd like to isolate the prismjs module for each test so that tests won't affect each other. But it seems no easy way. any ideas? @KyleAMathews

[KyleAMathews]

Eh, dunno. Not enough of a Jest expert to know how to do this.

[KyleAMathews]

Ooo… but now I am :-) https://facebook.github.io/jest/docs/en/configuration.html#resetmodules-boolean

[KyleAMathews]

jest.resetModules()

[vincentbel]

@KyleAMathews Works great! and it's ready to review now.

[gatsbybot]

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[gatsbybot]

Deploy preview ready!

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[gatsbybot]

Deploy preview ready!

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[gatsbybot]

Deploy preview ready!

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[vincentbel]

@KyleAMathews Besides, I think these tests are some as:

gatsby/packages/gatsby-remark-prismjs/src/__tests__/parse-line-number-range.js

Lines 4 to 43 in ca12b4d

	it(`parses numeric ranges from the languages variable`, () => {
	expect(parseLineNumberRange(`jsx{1,5,7-8}`).highlightLines).toEqual([
	1,
	5,
	7,
	8,
	])
	expect(parseLineNumberRange(`javascript{7-8}`).highlightLines).toEqual([
	7,
	8,
	])
	expect(parseLineNumberRange(`javascript{7..8}`).highlightLines).toEqual([
	7,
	8,
	])
	expect(parseLineNumberRange(`javascript{2}`).highlightLines).toEqual([2])
	expect(parseLineNumberRange(`javascript{2,4-5}`).highlightLines).toEqual([
	2,
	4,
	5,
	])
	})
	it(`ignores negative numbers`, () => {
	expect(parseLineNumberRange(`jsx{-1,1,5,7-8}`).highlightLines).toEqual([
	1,
	5,
	7,
	8,
	])
	expect(parseLineNumberRange(`jsx{-1..4}`).highlightLines).toEqual([
	1,
	2,
	3,
	4,
	])
	})
	it(`handles bad inputs`, () => {
	expect(parseLineNumberRange(`jsx{-1`).highlightLines).toEqual([])
	expect(parseLineNumberRange(`jsx{-1....`).highlightLines).toEqual([])
	})

Why duplicate here?

[KyleAMathews]

If there's duplicates, feel free to PR them away!

[KyleAMathews]

Thanks! Merging!