docs(04/2015): mark day as solved

Author: manhunto

readme.md

@@ -90,7 +90,7 @@
 | [Day 1: Not Quite Lisp](src/solutions/year2015/day01.rs)                         |   ⭐⭐   |            0.013 |            0.001 |
 | [Day 2: I Was Told There Would Be No Math](src/solutions/year2015/day02.rs)      |   ⭐⭐   |            0.074 |            0.105 |
 | [Day 3: Perfectly Spherical Houses in a Vacuum](src/solutions/year2015/day03.rs) |   ⭐⭐   |            0.358 |            0.373 |
-| [Day 4: The Ideal Stocking Stuffer](src/solutions/year2015/day04.rs)             |        |                - |                - |
+| [Day 4: The Ideal Stocking Stuffer](src/solutions/year2015/day04.rs)             |   ⭐    |           66.769 |                - |
 
 # TODO