[Ballista] Remove hard coded ballista versions in scripts

[msathis]

Which issue does this PR close?

Closes #32 .

Rationale for this change

Versions are hardcoded in too many places.

What changes are included in this PR?

Greps version from ballista/rust/core/Cargo.toml and exports it, so other scripts can use it.

Are there any user-facing changes?

N/A

[codecov-commenter]

Codecov Report

Merging #49 (4a99347) into master (3cb83fe) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #49   +/-   ##
=======================================
  Coverage   76.24%   76.24%           
=======================================
  Files         134      134           
  Lines       23051    23051           
=======================================
  Hits        17576    17576           
  Misses       5475     5475           

Impacted Files	Coverage Δ
benchmarks/src/bin/tpch.rs	35.15% <0.00%> (ø)
benchmarks/src/bin/nyctaxi.rs	0.00% <0.00%> (ø)

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[andygrove]

LGTM. I tested locally and integration tests passed.