ARROW-2488: [C++] Add Boost 1.67 and 1.68 as recognized versions

[wesm]

I added 1.68 so we won't have to do this again for a little while

[codecov-io]

Codecov Report

Merging #2136 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2136      +/-   ##
==========================================
- Coverage   86.41%   86.39%   -0.03%     
==========================================
  Files         242      230      -12     
  Lines       41496    40604     -892     
==========================================
- Hits        35859    35080     -779     
+ Misses       5637     5524     -113

Impacted Files	Coverage Δ
rust/src/record_batch.rs
rust/src/array.rs
rust/src/bitmap.rs
rust/src/datatypes.rs
rust/src/error.rs
rust/src/memory.rs
rust/src/lib.rs
rust/src/list.rs
rust/src/builder.rs
rust/src/list_builder.rs
... and 2 more

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[xhochy]

+1, LGTM