Move Gigasecond to the second exercise

Based on conversation in #112,
and by looking at the solutions that students have provided, the
Gigasecond exercise is very straightforward. Other than introducing
Crates it doesn't teach much about Rust.

Author: whit0694

config.json

@@ -5,14 +5,14 @@
   "active": true,
   "problems": [
     "hello-world",
+    "gigasecond",
     "leap",
     "anagram",
     "difference-of-squares",
     "sieve",
     "allergies",
     "word-count",
     "hamming",
-    "gigasecond",
     "rna-transcription",
     "nucleotide-count",
     "nucleotide-codons",