bracket notation fixes

[rjbaucells]

Changes:

• Updated "github.com/PaesslerAG/gval" to v1.2.2
• Set GO version to 1.15, same version used in "github.com/PaesslerAG/gval"
• fixed the following scenarios according to JSONPath Comparison:

Bracket notation with number -1

test: negativ select array
path: $[-1]
data: [7, "hey"]
result: "hey"

Bracket notation with number -1 on empty array

test: negativ select on short array
path: $[-2]
data: [7]
result: nil

Bracket notation with number on short array

test: simple select out of bounds
path: $[1]
data: ["hey"]
result: nil

[generikvault]

thank you