Data location must be “memory” for return parameter in function, but none was given

You should add memory keyword for string parameter, which was introduced in solidity version 0.5.0

As per the documentation:

Explicit data location for all variables of struct, array or mapping types is now mandatory. This is also applied to function parameters and return variables. For example, change uint[] x = m_x to uint[] storage x = m_x, and function f(uint[][] x) to function f(uint[][] memory x) where memory is the data location and might be replaced by storage or calldata accordingly. Note that external functions require parameters with a data location of calldata.

Corrected code

contract MyContract {
    string value;

    function get() public view returns (string memory) {
        return value;
    }

    function set(string memory _value) public {
        value = _value;
    }

    constructor() public {
        value = "myValue";
    }
}

Refer to official documentation on breaking changes made in version 0.5.0