Update refs and tests to match latest NuGet.

[JimBobSquarePants]

Prerequisites

• [x] I have written a descriptive pull-request title
• [x] I have verified that there are no overlapping pull-requests open
• [x] I have verified that I am following matches the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules :cop:.
• [x] I have provided test coverage for my change (where applicable)

Description

Updated to Fonts 1.0.0-beta15 Updated to ImageSharp 1.0.3

I've added a new test for fallback fonts and updated all the reference examples for text rendering. Both our vertical alignment and our line height is now correctly calculated which lead to differences.

[codecov[bot]]

Codecov Report

Merging #135 (8c4c7e3) into master (1e1d6ea) will decrease coverage by 0.03%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #135      +/-   ##
==========================================
- Coverage   69.38%   69.35%   -0.04%     
==========================================
  Files          89       89              
  Lines        5142     5142              
  Branches     1051     1051              
==========================================
- Hits         3568     3566       -2     
- Misses       1358     1359       +1     
- Partials      216      217       +1     

Flag	Coverage Δ
unittests	69.35% <ø> (-0.04%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...ssing/Processors/Text/DrawTextProcessor{TPixel}.cs	97.15% <0.00%> (-0.95%)	:arrow_down:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update d7b5c69...8c4c7e3. Read the comment docs.